This invention relates generally to a liquid crystal imaging system and, more particularly, to an imaging system wherein a layer of a homeotropically aligned nematic liquid crystalline material is the active element of an imaging member.
The name "liquid crystals" has become generic to liquid crystalline materials which exhibit dual physical characteristics, some of which are typically associated with liquids and others which are typically unique to solids. Liquid crystals exhibit mechanical characteristics, such as viscosities, which are ordinarily associated with liquids. The optical scattering and transmission characteristics of liquid crystals are similar to those characteristics ordinarily unique to solids. In liquids or fluids, the molecules are typically randomly distributed and oriented throughout the mass of the substance. Conversely, in crystalline solids, the molecules and/or atoms are generally rigidly oriented and arranged in a specific crystalline structure. Liquid crystals resemble solid crystals in that the molecules of the liquid crystalline substances are regularly oriented in a fashion analogous to but less extensive than the molecular orientation and structure in a crystalline solid. Many substances have been found to exhibit liquid crystalline characteristics in a relatively narrow temperature range; but below such temperature ranges, the substances typically appear as crystalline solids and above such temperature ranges, they typically appear as liquids.
Liquid crystals are known to appear in three different forms: the smectic, nematic and cholesteric forms. These structural forms are sometimes referred to as mesophases thereby indicating that they are states of matter intermediate between the liquid and crystalline states. The three mesophase forms of liquid crystals mentioned above are characterized by different physical structures wherein the molecules are arranged in a manner which is unique to each of the three mesomorphic structures. Each of these three structures is well known in the liquid crystal art.
It is well known in the art that liquid crystalline materials can be used in imaging techniques which utilize their response to electrical fields. U.S. Pat. No. 3,687,515 to Haas et al discloses an electrooptic system wherein a layer of a nematic liquid crystalline composition which is optically uniaxial with the optic axis normal to the plane of the layer has an electrical field applied perpendicular to the optic axis of the composition layer thereby inducing optical biaxiality in the composition layer. This patent teaches an imaging system which exploits the optic retardation accompanying a field induced change from the uniaxial to the biaxial state. It is also disclosed therein that a layer of a nematic liquid crystalline material may be more readily made to adopt a homeotropically aligned texture state, i.e., wherein the major molecular axes of a substantial portion of the molecules are aligned substantially perpendicular to the plane of the layer, when deposited on a substrate by incorporating a surfactant additive material in the nematic liquid crystalline material. Other techniques for inducing the homeotropic texture state in layers of nematic liquid crystalline materials residing on substrates are also known in the art such as, for example, rubbing the surface of the substrate prior to depositing the nematogenic material thereupon or treating the surface with a material such as lecithin (see, for example, U.S. Pat. No. 3,597,150). For a detailed description of the nematic homeotropic texture see Gray, G. W. Molecular Structure and the Properties of Liquid Crystals, Academic Press, London, 1962.
There has also been disclosed in the art a device wherein a pair of transparent electrodes form a sandwich around a layer of homeotropically aligned nematic liquid crystalline material residing on a photoconductive insulating layer. It is disclosed that the effects caused by the application of electrical fields across the liquid crystal layer-photoconductive layer combination may be exploited, for example, to transform a black and white image into a color image. See Assouline et al, C.R. Acad. Sci., Paris, t. 274, 692 -- Serie B (6 Mar. 1972).
The present invention relates to a novel and advantageous liquid crystal imaging system wherein the optic retardation accompanying an electrical field induced change from the uniaxial to the biaxial state in selective areas of a nematic liquid crystalline layer is exploited.